Technical Field
The present invention relates to photovoltaic devices, and more particularly to devices and methods for formation for a photovoltaic device that includes a digital alloy.
Description of the Related Art
In Si heterojunction solar cells (also known as heterojunction with intrinsic thin-layer (HIT) cells), diffused junctions, which are employed in conventional Si solar cells, are replaced by amorphous Si (a-Si:H)/crystalline Si (c-Si) heterojunctions. This offers an advantage of low-temperature, low-cost processing (due the low-cost and low-temperature deposition of a-Si:H at ˜200 degrees C. compared to that of >600 degrees C. for diffusion). In addition, high open circuit voltages due to the excellent surface passivation properties of a-Si:H are also obtained. This concept may also be applied to Ge substrates to obtain higher short circuit currents due to a stronger absorption of sunlight in Ge. However, the high cost of Ge substrates counters such advantages.
To address this issue, rapid and low-cost epitaxial growth of Ge on Si substrates has been employed to fabricate Ge heterojunction solar cells. This employs growth of a graded buffer layer comprised of SiGe on the Si substrate in which the concentration of Ge is gradually increased to avoid excessive accumulated strain due to lattice mismatch between Si and Ge. This prevents high dislocation density or the disruption of the epitaxy; however, thermal generation and recombination of minority carriers occurs in such a buffer layer. This is to the first order proportional to the thickness of the buffer layer, and contributes to the efficiency loss of the solar cell accordingly.